Silicon carbide (SiC) is promising to be a next-generation semiconductor material that can achieve high-voltage, low-loss semiconductor devices. Particularly promising semiconductor devices including silicon carbide are insulated gate semiconductor devices such as metal-oxide-semiconductor field effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs) that enable switching operations. The insulated gate semiconductor devices need to have characteristics, namely, normally-off characteristics in which no current passes when voltage is not applied to a gate.
To be used as the semiconductor devices having the normally-off characteristics, semiconductor devices need to have a high threshold voltage (Vth) to some extent, the threshold voltage being a gate voltage when ON-state current starts to flow. For example, IGBTs, which are commonly and commercially available and include silicon (Si), have a typical threshold voltage of 5 V. A high threshold voltage of at least several electron volts or greater is needed in such manner in consideration of a malfunction and an operation at high temperature.
The threshold voltage is greatly influenced by a fixed charge in a gate insulating film and interface traps at a so-called MOS interface between silicon carbide and the gate insulating film. When silicon carbide is used in comparison with silicon (Si) typically used as a material for conventional semiconductor devices, more interface traps occurring at the MOS interface and poor quality of the MOS interface have been known.
Many interface traps having an energy level of 0.2 eV or shallower from a conduction band end of silicon carbide are at the MOS interface in which a channel is formed during ON operations, thereby increasing an ON resistance, namely, an ON-state loss due to a decrease in a channel conductance (channel mobility). Thus, a reduction of the interface traps has been actively developed. For example, a technique for reducing interface traps at an MOS interface by a heat treatment in a hydrogen (H2) gas (hydrogen annealing), a heat treatment in a nitrogen monoxide (NO) gas or a dinitrogen monoxide (N2O) gas (nitrogen annealing), and a heat treatment in a phosphorus oxychloride (POCl3) gas (POCl3 annealing), and a technique for increasing channel mobility are disclosed (see Patent Document 1, for example).